Materials for the direct capture of carbon dioxide from atmospheric air

ABSTRACT

The invention relates to a method to produce a particulate activated carbon material for capturing CO2 from air,wherein the particulate activated carbon is impregnated with alkali carbonate salt such as K2CO3; and wherein the impregnated particulate activated carbon either has, determined using nitrogen adsorption methods, a pore volume of at least 0.10 cm3/g for pore sizes of at least 5 nm and a pore volume of at most 0.30 cm3/g for pore sizes of less than 2 nm or is based on a mixture of different alkali carbonate salts, or has a particular pore surface for pore sizes in the range of 2 nm-50 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/EP2018/080655, filed on Nov. 8, 2018, which claims priority fromEuropean Patent Application No. 17201080.3 filed on Nov. 10, 2017, andEuropean Patent Application No. 18157300.7, filed on Feb. 19, 2018.

TECHNICAL FIELD

The present invention relates to materials and processes for the directcapture of carbon dioxide from atmospheric air.

PRIOR ART

In order to limit climate change to acceptable levels it is necessary toreduce carbon dioxide (CO₂) emissions in the near future to zero as wellas to achieve net carbon dioxide removal from the atmosphere. More than87% of modeling scenarios of the Intergovernmental Panel on ClimateChange (“IPCC”) consistent with 2° C. of warming involve large-scaledeployments of CO₂ removal technologies. Direct air capture of CO₂(“DAC”) can be an important technology in order to contribute toemission reduction through for example the supply of atmospheric CO₂ forthe synthesis of renewable materials or fuels, as described inWO2016/161998. Further DAC can be important in order reduce theatmospheric CO₂ content through combination of DAC with a safe andpermanent method to store the CO₂, e.g. through undergroundmineralization.

Different technologies for DAC have been described, e.g. DAC by liquidsolutions of sodium hydroxide as described in US2010034724, DAC by basicion exchange resins as described in US2009120288, DAC by amine-modifiedadsorbents as described in WO2012/168346, as well as DAC by K₂CO₃impregnated supports as described in Veselovskaya et al. inInternational Journal of Greenhouse Gas Control 17 (2013) 332-340.Generally application of K₂CO₃ impregnated supports for CO₂ capture fromflue gases and air is well known.

U.S. Pat. No. 3,511,595 describes a process for removing CO₂ from air ofenclosed spaces based on particulate carriers of large surface areahaving distributed thereon an alkali metal carbonate, specificallyK₂CO₃.

WO-A-2016/185387 describes a process for CO₂ capture from a gas streambased on a sorbent material having an alumina or a silica aluminasupport and where potassium carbonate (K₂CO₃) is impregnated on thesupport. The support has a surface area of 170-550 m²/g, a pore volumeof 0.18-0.95 cm³/g and a pore size of 10-30 nm.

WO-A-2014/012963 describes a material for reversibly adsorbing CO₂ froma gas mixture, specifically air, where the material is composed of asolid material made of ceramic, zeolite or activated carbon, having asurface area of 150-600 m²/g, containing a salt capable of reacting withwater and CO₂ to form bicarbonate, where the salt is of an alkali metalor an alkaline earth metal.

US-A-2006/0148642 describes a material for CO₂ capture from flue gas offossil fuel fired power plants where the material is composed of 70% orless or a reactive compound which is capable to be converted to a metalcarbonate, 70% or less of a solid porous non-metallic support and 70% orless of an inorganic binder, where the solid support is among the groupof ceramic-like materials, including alumina, silica, magnesia,zirconia, titania, natural and synthesized zeolites, diatomaceous earthand carbon molecular sieves, having a surface area greater than 50 m²/g.

U.S. Pat. No. 4,433,981 describes a process for removing CO₂ from a gasstream providing a porous alumina impregnated with a sodium or potassiumsalt, where the support has a surface area of at least 100 m²/g.

U.S. Pat. No. 4,493,715 describes a method of removing CO₂ from olefincontaining gas streams based on an alumina support having a surface areaof greater than 50 m²/g, preferably greater than 200 m²/g and where thesupport is impregnated with an alkali metal compound.

U.S. Pat. No. 6,280,503 describes a process for capturing CO₂ at hightemperatures of 300-500° C. based on a sorbent containing a support ofmagnesium oxide as well as being impregnated with an alkali metal, e.g.potassium.

Veselovskaya et al. in International Journal of Greenhouse Gas Control17 (2013) 332-340 describe an adsorbent for CO₂ capture from air, whichis composed of an alumina support impregnated with K₂CO₃. The surfacearea of the pristine support was 240 m²/g, the pore volume was 0.59cm³/g and the average pore size 10 nm.

Guo et al. in Chem. Eng. Technol. 2015, 38, No. 5, 891-899 describeactivated carbon impregnated with K₂CO₃ for CO₂ removal from confinedspaces having low CO₂ concentrations. The support had a surface area of579.57 m² and a pore volume of 0.33 cm³/g. The authors concluded thatmainly the pores in the size range of 1-10 nm are filled with K₂CO₃,where the pores in the size range of 10-100 nm are not filled withK₂CO₃.

Zhao et al. in Chemical Engineering Journal 254 (2014) 524-530 describeK₂CO₃ modified activated carbons for CO₂ removal from confined spaces.The surface area of the pristine support was 957 m², the pore volume was0.362 cm³/g and the pore size was not disclosed.

Zhao et al. in Ind. Eng. Chem. Res. 2011, 50, 4464-4470 describesdifferent supported K₂CO₃ sorbents for CO₂ capture from gas streamshaving a CO₂ concentration of 15%, 15% H₂O and balance N₂. The authorsconclude that the sorbents based on alumina are better than those basedon activated carbon, since first allow for uniform distribution of K₂CO₃on the support.

Hayashi et al. in Ind. Eng. Chem. Res. 1998, 37, 185-191 describedifferent K₂CO₃ modified activated carbon for CO₂ capture from gasstreams containing 13.8% CO₂. The authors conclude that the macroporesof activated carbons, thus the pores having a pore size greater than 50nm, are filled with K₂CO₃ during impregnation and contribute in such tooffer pore volume for the CO₂ capture process.

Prajapati et al. in Energy Fuels 2016, 30, 10758-10769 describesK₂CO₃/activated sorbent for CO₂ capture from gas streams with differentCO₂ concentrations from 10-100% in a fluidized bed arrangement.

Acar et al in Catalysis Today 301, pages 112-124 discuss CO₂ Adsorptionover modified AC samples and present a new methodology for determiningselectivity. Activated carbon (AC) based adsorbents having high andstable CO₂ adsorption capacity with enhanced CO₂ selectivity in presenceof CH₄ were developed. Alkali modified AC samples were prepared, theirCO₂ adsorption capacities were measured, a new methodology for selectiveadsorption capacity determination under multicomponent gas mixture flowwas developed, and the results were analyzed to determine thepreparation procedure yielding optimum adsorbent design. Two groups ofadsorbents were prepared by K₂CO₃ impregnation on air and HNO3 oxidizedforms of a commercial AC followed by calcination at varioustemperatures. The resulting adsorbents were named according tocalcination temperatures as ACxK-calT. The highest CO₂ adsorptioncapacity was measured on AC3K-300 sample as 110 mg/g. adsorbent at 1000mbar CO₂ and 25° C. CO₂ adsorption was confirmed reversible, whereas CH₄adsorption was found partially irreversible. The highest mass basedCO₂:CH₄ selectivity, ca. 3.7, was achieved over AC2K-200 at 25° C. forthe 50% CO₂-50% CH mixture. AC2K-200 was further tested at higher totalpressures, for 0-5000 mbar pressure range, at 25° C. CO₂ adsorptioncapacity was measured as 197 mg/g. adsorbent at 5000 mbar CO₂. AmongLangmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models, D-Rwas found to be the most successful one explaining CO₂ adsorptionbehavior of AC samples.

Zhao et al in Chem. Eng. J. 254 (2014) 524-530 discuss the carbonationbehavior of K₂CO₃/AC in low reaction temperature and CO₂ concentration.The carbonation behavior of K₂CO₃/AC was investigated in the conditionof low reaction temperatures of 20-60° C. and low CO₂ concentrations of0-4% in TGA. The reaction path of K₂CO₃/AC consists of two steps as thatthe hydration reaction occurs first to form K₂CO₃ 1.5 H2O and K4H2(CO₃)₃1.5H₂O, then KHCO₃ is produced rapidly. Besides the carbonationreaction, the adsorption process exists either. More K₂CO₃ will beconverted to KHCO₃ for the sorbent with low K₂CO₃ loadings or in theconditions of high reaction temperature and low H₂O concentration. MoreK₄H₂(CO₃)3 1.5H2O and K₂CO₃1.5H₂O will be formed for the sorbent withhigh K₂CO₃ loadings or in the conditions of low reaction temperature andhigh H₂O concentration. The effect of the CO₂ concentration is notsignificant on the carbonation reaction paths of K₂CO₃/AC. On thecontrary, the Relative Humidity (RH) plays an important role in thisprocess. Lee et al in Environ. Sci. Technol., 2008, 42 (8), pp 2736-2741discuss the development of regenerable MgO-based sorbent promoted withK₂CO₃ for CO₂ capture at low temperatures. To improve their CO₂absorption capacity, alkali-based sorbents prepared by impregnation andwet mixing method of potassium carbonate on supports such as activatedcarbon and MgO (KACI30, KACP30, KMgI30, and KMgP30), were investigatedin a fixed bed reactor (CO₂ absorption at 50-100° C. and regeneration at150-400° C.). Total CO₂ capture capacities of KMgI30-500 and KMgP30-500were 178.6 and 197.6 mg CO₂/g sorbent, respectively, in the presence of11 vol % H2O even at 50° C. The large amount of CO₂ capture capacity ofKMgP30-500 and KMgI30-500 could be explained by the fact that MgOitself, as well as K₂CO₃, could absorb CO₂ in the presence of watervapor even at low temperatures. In particular, water vapor plays animportant role in the CO₂ absorption of MgO and KMgI30-500 even at lowtemperatures below 60° C., in marked contrast to MgO and CaO which canabsorb CO₂ at high temperatures. The CO₂ capture capacity of theKMgI30-300 sorbent, however, was less than that of KMgI30-500 due to theformation of Mg(OH)₂ which did not absorb CO₂. MgO based-sorbentspromoted with K₂CO₃ after CO₂ absorption formed new structures such asK₂Mg(CO₃)₂ and K₂Mg(CO₃)₂.4(H₂O), unlike KACI30 which showed only theKHCO₃ crystal structure. The new Mg-based sorbents promoted with K₂CO₃showed excellent characteristics in that it could satisfy a large amountof CO₂ absorption at low temperatures, a high CO₂ absorption rate, andfast and complete regeneration.

Shigemoto et al in Energy Fuels, 2006, 20 (2), pp 721-726 discuss thematerial balance and energy consumption for CO₂ recovery from moist fluegas employing K₂CO₃-on-Activated Carbon and its evaluation for practicaladaptation. Potassium carbonate supported on an activated carbon hasbeen proposed as an efficient sorbent to recover CO₂ from moist fluegases. As a characteristic of the present CO₂ sorption process, whichcan be described as K₂CO₃.1.5H₂O+CO₂=2KHCO₃+0.5H₂O, moisture in the feedgases had no influence on the CO₂ sorption. By the temperature-swingoperation of a fixed-bed, the CO₂ recovery was achieved as follows:carbon dioxide in moist flue gases at around 363 K was sorbed by theK₂CO₃ sorbent, followed by steam flushing at 433 K to release the CO₂,and then cooling the sorbent for the next CO₂ sorption. In the presentstudy employing a bench-scale apparatus, the material (CO₂ and H₂O)balances, together with those of heat during each step, were measured toelucidate the CO₂ sorption/release and the cooling behaviors. Toevaluate the practical adaptability of this process, the heatconsumption for the CO₂ recovery on a commercial-scale was estimated.When compared with that for other processes such as the conventionalamine process, it provided a remarkable energy-conservative effect. Thecost for the CO₂ recovery by K₂CO₃-on-activated carbon is alsodiscussed.

SUMMARY OF THE INVENTION

Prior art does not disclose parameters of support material for alkalicarbonates such as K₂CO₃ which are optimal for CO₂ capture from air. Itis thus one object of the present invention to propose improvedmaterials for the direct capture of carbon dioxide from atmospheric airas well as uses thereof.

According to a first aspect of the present invention a method for makinga particulate activated carbon material for capturing CO₂ from air isproposed, wherein the particulate activated carbon is impregnated withat least one alkali carbonate salt selected from the group consistingof: K₂CO₃, Li₂CO₃, Na₂CO₃ as well as mixed salts thereof.

According to the proposed method, at least one alkali carbonate saltselected from the group consisting of: K₂CO₃, Li₂CO₃, Na₂CO₃ as well asmixed salts thereof is dissolved in a solvent, and, at the same time orfollowing dissolution of the alkali carbonate salt, pristine particulateactivated carbon, if need be after drying and/or purification,preferably non-oxidized, having a specific surface area, determinedusing nitrogen adsorption methods as described in ISO 15901-2 and ISO15901-3 and according to the t-plot method, of at least 80 m2/g in thepore size range of more than 2 nm to at most 50 nm, is added to form asuspension,

and wherein subsequently at least the solid fraction is isolated and/ordried by evaporation to obtain the impregnated particulate activatedcarbon.

Activated carbon is available in a large variety of forms according tothe raw material from which it is obtained, according to the processused for its activation, and according to the final form that is givento the final product. In particular activated carbon is used indifferent applications according to the specific pore structurecharacterizing its internal space. We have investigated the correlationbetween the pore structure of activated carbon and the efficacy of theactivated carbon within a given formulation with alkali carbonate salt,in particular with K₂CO₃, in the process of direct CO₂ capture from air.We can show that for activated carbon supports impregnated with alkalicarbonate salt, in particular K₂CO₃, the CO₂ adsorption capacities fromambient air increases with increasing mesopore surface. In particularactivated carbons having mesopore surfaces above 80 m2/g are especiallyapt as formulation ingredient.

The mesopore range is preferably 80-600 m2/g, and most preferably 80-400m2 g, for the mesopore surface of the pristine activated carboningredient used in the formulation of the sorbent.

According to a first preferred embodiment of the method, the solvent iswater, preferably deionized water.

Further preferably the concentration of the alkali carbonate salt is 1-8mmol (total) alkali carbonate salt per ml solvent (e.g. water),preferably 1.5-4.5 mmol/ml solvent (water).

Preferably the pristine, preferably non-oxidized, particulate activatedcarbon is added to the solution under stirring.

Preferably the solvent is held at least for a certain amount ofimpregnation time, preferably over the full impregnation time, at atemperature in the range of 5-40° C., most preferably at a temperaturein the range of 20-30° C., and/or the impregnation is carried out for atime span in the range of 30 minutes-100 hours, most preferably in therange of 2-40 h or 2.5-3 h or 6 hours-40 hours, and wherein subsequentlyat least the solid fraction is isolated and/or dried by evaporation,preferably vacuum evaporation.

Preferably the pristine particulate activated carbon support has a porevolume of at least 0.1 cm3/g and preferably at most of 1.5 cm3/g in thepore size range more than 2 nm to at most 50 nm and/or a specificsurface area of 80-600 m2/g, preferably 80-400 m2/g in the pore sizerange of more than 2 nm to at most 50 nm, in each case determined usingnitrogen adsorption methods according to ISO 15901-2 and ISO 15901-3 andaccording to the t-plot method.

To obtain a good sorbent it is important that the alkali carbonate salt(K₂CO₃) solution enters the pores of the support and infiltrates a largeinternal surface area. Capillary forces can be assumed to be a majordriving force for this process. However, before the infiltration thepores are usually filled with air that needs to be replaced and thattakes time to diffuse out of the porous support. Usually, rather longsoaking times >12 h are used for the impregnation of activated carbonwith alkali carbonate salt, e.g. K₂CO₃. The infiltration can beaccelerated by the application of vacuum to remove the entrapped airfollowed by a return to atmospheric pressure to push the solution intothe pores.

E.g. the application of a vacuum of 100 mbar for 2 times 5 min followedby a return to ambient pressure at the beginning of a 3 h impregnationperiod allowed improving the CO₂-adsorption capacity of the sorbent by afactor of 2.4 for a 180-min adsorption experiment.

Therefore according to a further preferred method the suspension issubjected to at least one time period with reduced pressure, preferablya vacuum of at most 300 mbar, preferably at most 200 mbar, mostpreferably in the range of 10-150 mbar, wherein that reduced pressuretime period is at least 60 seconds, preferably at least 2 minutes, mostpreferably 3-20 or 5-10 minutes, and wherein before isolation of theimpregnated particulate activated carbon the suspension is returned toambient pressure for a time period of at least 60 seconds, whereinpreferably at least two such cycles including a time period of reducedpressure of at least 60 seconds, preferably of at least 2 min, and afollowing time period of at least 60 seconds, preferably of at least 2min, of ambient pressure are carried out, and wherein further preferablythe total impregnation period before isolation is in the range of 2-5hours, preferably in the range of 2.5-3.5 hours.

According to another aspect of the present invention it relates toimpregnated particulate activated carbon, preferably obtained orobtainable using the method as described above. Herein it was found thatactivated carbon (AC) supports featuring a large share of pore volumeand/or pore surface in the pore size range of 2-50 nm (mesoporous range)and preferably 50-1'000 nm (small macroporous range) are especiallyfeasible support materials. Activated carbon materials have been studiedas supports for K₂CO₃ modification, however, the importance ofmesoporosity for CO₂ capture from air has not been disclosed.Mesoporosity has been described in the prior art in the context ofalumina supports, however, alumina based K₂CO₃ sorbents require higherregeneration temperature than carbon based supports and are thus notuseful for the CO₂ capture from air process described herein. Thealumina surface together with K₂CO₃ forms mixed salts, e.g.KAl(CO₃)²⁻(OH)⁻ ₂, resulting in different reaction mechanisms with CO₂in turn requiring a higher regeneration temperature in comparison toK₂CO₃ as such. In contrary, the surface of activated carbons is largelygraphite, which does not react chemically with K₂CO₃ and acts as achemically neutral carrier. The findings herein are not limited to K₂CO₃but do also apply to the named other alkali metal salts, i.e. Na₂CO₃,Li₂CO₃, as well as mixed forms thereof such as NaKCO₃.

More specifically, the present invention relates to a particulateactivated carbon material for capturing CO₂ from air, which isimpregnated with at least one alkali carbonate salt selected from thegroup consisting of: K₂CO₃, Li₂CO₃, Na₂CO₃ as well as mixed saltsthereof, preferably obtained or obtainable using the method as describedabove.

The impregnated particulate activated carbon has, according to oneaspect of the present invention and determined using nitrogen adsorptionmethods as detailed in the experimental section and as described in ISO15901-2 and ISO 15901 and according to the QSDFT calculation scheme, apore volume of at least 0.10 cm³/g for pore sizes of at least 5 nm and apore volume of at most 0.30 cm³/g for pore sizes of less than 5 nm.

The impregnated particulate activated carbon has, according to anotheralternative or cumulative aspect of the present invention, the featurethat the particulate activated carbon is impregnated with a mixture ofat least two different alkali carbonate salts selected from the groupconsisting of: K₂CO₃, Li₂CO₃, Na₂CO₃. As detailed and shown in theexperimental section surprisingly the high proportion of mesopores andsmall macropores compared with micropores leads to a particularlyefficient carbon dioxide capture property of the particulate activatedcarbon material. Furthermore in contrast to silica or alumina supportmaterials, the essentially inert activated carbon carrier allows forlower temperatures and/or less vacuum during the desorption in thecorresponding cycles.

As detailed and shown in the experimental section further surprisinglythe carbon dioxide capture property of the AC/K₂CO₃ sorbent materialtend to increase with the increase of the mesopore surface and volume,while they tend to decrease with the increase of the micropore surfaceand volume, where the pore surface and the pore volume are measured bynitrogen adsorption. Furthermore in contrast to silica or aluminasupport materials, the essentially inert activated carbon carrier allowsfor lower temperatures and/or less vacuum during desorption in thecorresponding cycles.

As also detailed and shown in the experimental section surprisingly amixture of at least two different alkali carbonate salts leads to aparticularly efficient carbon dioxide capture property of theparticulate activated carbon material increasing capture efficiency andreducing the desorption temperature and in such reducing the energyconsumption and increasing capture efficiency.

According to a preferred embodiment the particulate activated carbon isimpregnated with a mixture of at least two different alkali carbonatesalts selected from the group consisting of: K₂CO₃, Li₂CO₃, Na₂CO₃, andan alkali carbonate salt with the smallest weight proportion in themixture is present in an amount of at least 5 weight %, preferably atleast 10 weight %, in each case with respect to the total of theimpregnating mixture of at least two alkali carbonate salts.

According to a another preferred embodiment the impregnating mixture ofat least two alkali carbonate salts comprises at least Na₂CO₃,preferably said mixture comprising or consisting of K₂CO₃ as well asNa₂CO₃, preferably in a weight ratio of K₂CO₃ to Na₂CO₃ in the range of95:5-5:95, more preferably in the range of 90:10-10:90, most preferablyin the range of 60:40-90:10.

Alternatively the mixtures can be characterized by way of the molarproportion of the respective cations. According to thischaracterization, preferably the alkali cation with the smallest molarproportion with respect to all cations in the mixed salt is at least 2%,preferably at least 5%, more preferably at least 10%. For the specificsituation of a mixed salt including or consisting of sodium andpotassium, the molar proportion between Na and K is preferably in therange of 24.7-0.07, preferably in the range of 11.7-0.14, mostpreferably in the range of 1.30-0.07.

In relation with the above mentioned prior art the following:

The formulations of the sorbent materials discussed by Acar et al. arebased on powderised and highly acid oxidized activated carbons andconsequently different from the ones that are used herein. In fact theCO₂ capacity described by Acar et al. is above the theoretical capacitypossible if carbonates are the only capture chemicals, indicating adifferent mechanism of carbon capture. Due to the low concentration ofCO₂ in air, the practical utilization of a sorbent requires very highvolume flows. The sorbent material described by Acar et al. is in theform of a powder expressly obtained from the crushing of pelletizedcarbon. According to Chen et al. (Chem et al. Powder technology 124(2002) 127) for an exemplary 27 μm powder one can use a pressure dropcoefficient of 125000 Pa·s/m2. Considering realistic conditions for thecapture of CO₂ from air, therefore a velocity through the bed of 0.2 m/sand a pressure drop of 150 Pa, such sorbent would require a bed heightof ca 6 mm, which would not be feasible for a technical implementation.On the other hand, given the above conditions feasible for the capturingof CO₂ from air, a packed bed of 20 mm thickness of the materialproposed by Acar et al. would generate a pressure drop of 500 Pa, thusgenerating prohibitively high energy costs. The above arguments provethat the material proposed by Acar et al. is not apt for capturing CO₂from ambient air. In fact, the authors claim that their task isobtaining materials that have a high capacity in terms of CO₂ adsorptionin the additional presence of methane, but not for the capture of CO₂from ambient air. The carbon capture device used by Acar et al. is atesting instrument (thermogravimetric analyzer), typically used inacademic laboratories in order to investigate phenomena, and cannot bescaled up to actually capture CO₂ from air.

While the sorbent material used by Zhao et al. is an activated carbonimpregnated with K₂CO₃ in order to capture CO₂ from air, the formulationherein comprises more specific properties of the activated carbonsupport, in particular the pore size distribution versus the totalspecific surface area as discussed in Zhao et al. The invention hereshows that activated carbon materials may have a large specific surfacearea but still not be apt as base for the formulation of a carboncapture composite material. The invention herein is the identificationof the physical properties that make a good sorbent material and the useof such properties to formulate a material that can work at increasedefficiency as respect to current K₂CO₃ formulations. At CO₂concentrations representative for capturing CO₂ from air, i.e. 400 ppmvor 4E-4 in the dimensionless form as in FIG. 3 shown in Zhao et al., theCO₂ capacity of the sorbent materials of Zhao et al. lie well below 0.25mmol CO₂/g, where herein capacities well above 1 mmol CO₂/g are shown,indicating that the pore properties in Zhao et al. are not optimized forcapturing CO₂ from air. The device used by Zhao et al. is a testingsystem rather than a carbon capture device. The purpose of a testingsystem is to verify a given property of the material rather than being atemplate for the operation of the materials in large scale.

The activated carbon sorbents used by Lee et al. (KACI30 and KACP30)show a high CO₂ total capacity (1.7 and 1.8 mmol/g) in a gas containing1 vol % of CO₂ after an exposure of ca. two hours. Considering that theconcentration of CO₂ in ambient air is 0.04 vol %, and that the reactionkinetic is first order and only depends on the CO₂ concentration,[Onishak et al, Kinetic of the reaction of CO₂ with solid K₂CO₃, in:73rd National AlChE Meeting, Minneapolis, USA, 1972], the kinetic of thereaction for capturing CO₂ from ambient air using the material proposedby Lee et al. would be correspondingly reduced by almost two orders ofmagnitude, rendering the reaction prohibitively slow, showing that thematerials in question were not suitable for direct capture of CO₂ fromair. Here we show CO₂ capacities of above 1 mmol CO₂/g achieved afterca. six hours, hence, significantly faster than Lee et al.

Shigemoto et al show that the sorbent material formed by impregnation ofactivated carbon with K₂CO₃ has a breakthrough after ca 1 h of exposureto a gas containing ca 10% v/v CO₂. Considering that air has aconcentration to CO₂ of ca 0.04% v/v and using the argument regardingreaction kinetics as shown above, then the breakthrough obtained if thematerial was used to capture CO₂ from ambient air would be longer byalmost three orders of magnitude, due to the slower kinetics of CO₂reaction, therefore rendering the material too slow in CO₂ capture fromambient air. In fact the authors have proposed the material for CO₂capture from flue gas. This shows that the material developed byShigemoto et al. is not apt for the application of direct CO₂ capturefrom air. Here we show CO₂ capacities of above 1 mmol CO₂/g achievedafter six hours, hence, significantly faster than Shigemoto et al.

As detailed and shown in Example 3 further surprisingly the carbondioxide capture property of the AC/K₂CO₃ sorbent material tends toincrease with the increase of the mesopore volume, while they tend todecrease with the increase of the micropore volume. Furthermore incontrast to silica or alumina support materials, the essentially inertactivated carbon carrier allows for lower temperatures and/or lessvacuum during desorption in the corresponding cycles.

According to a first preferred embodiment, the alkali carbonate (e.g.K₂CO₃) impregnated particulate activated carbon has, determined usingnitrogen absorption methods, as described in ISO 15901-2 and ISO15901-3, and according to the QSDFT calculation scheme as detailed inExample 3, a pore volume of at least 0.04 cm³/g, 0.05 cm³/g or 0.1cm³/g, preferably in the range of 0.05-2.2 or 0.1-2.2 or 0.2-1.5 cm³/gfor pore sizes above 2 or above 5 nm or in the range of 2-50 or 5-50 nm.

According to another preferred embodiment, the alkali carbonate (e.g.K₂CO₃) impregnated particulate activated carbon has a pore volume of atleast 0.05 cm³/g for pore sizes in the range of 50-1'000 nm, asdetermined using mercury porosimetry analysis as described in ISO15901-1 and detailed in Example 6.

According to another preferred embodiment, the alkali carbonate (e.g.K₂CO₃) impregnated particulate activated carbon has a pore volume,determined as detailed in Example 3, of at most 0.4 or 0.25 cm³/g or 0.2cm³/g or in the range of 0.05-0.2 cm³/g or 0.05-0.15 cm³/g for poresizes of less than 2 or less than 5 nm.

According to another preferred embodiment, the alkali carbonate (e.g.K₂CO₃) impregnated particulate activated carbon has a pore surface of atleast 20 m2/g or 35 m2/g, preferably in the range of 40-250 or 45-200m2/g for pore sizes above 2 nm or in the range of 2-50 nm, determinedusing nitrogen adsorption methods as described in ISO 15901-2 and ISO15901 and according to the QSDFT calculation scheme.

The K₂CO₃ impregnated particulate activated carbon preferably has, asdetailed in Example 3, a pore surface of at least 20 m²/g, preferably inthe range of 25-500 40-400 or or 50-400 m²/g for pore sizes of at least2 nm or at least 5 nm or in the range of 2-50 or 5-50 nm.

The alkali carbonate (e.g. K₂CO₃) impregnated particulate activatedcarbon can have pore surface of at most 900 or 500 m²/g or in the rangeof 150-500 or 100-400 m²/g for pore sizes of less than 2 nm or less than5 nm determined using methods as described in Example 3.

The impregnated particulate activated carbon preferably has a BETsurface area determined according to ISO 9277 in the range of 100-800m²/g, or of less than 1000 or less than 500 m²/g.

The alkali carbonate (e.g. K₂CO₃) impregnated particulate activatedcarbon in addition to that typically has a tapped density measuredaccording to the method described in Example 2 in the range of 300-800kg/m³, preferably in the range of 400-600 kg/m².

The impregnated particulate activated carbon can have a particle size(preferably expressed as D50 determined in accordance with ISO 9276-2(2014)) in the range of 0.1-8 mm, preferably in the range of 0.2-4.5 mm,most preferably in the range of 0.5-1.5 mm and/or in the range of mesh(ASTM) 3-140, preferably 4-50.

The impregnated particulate activated carbon preferably contains atleast 10% by weight of alkali carbonate (e.g. K₂CO₃), preferably atleast 20% by weight, or at least 30% by weight, most preferably in therange of 25-45% by weight.

The impregnated particulate activated carbon according to the inventionpreferably has an average carbon dioxide capacity, at 30° C., 60%relative humidity and 450 ppmv carbon dioxide concentration in air oranother gas or gas mixture after 1000 minutes adsorption of in the rangeof 0.5-5 mmol/g, preferably in the range of 1-2.5 mmol/g and/or after180 minutes adsorption of in the range of 0.25-2 mmol/g, preferably inthe range of 0.5-2 mmol/g or in the range of 0.6-1.5 mmol/g.

According to another preferred embodiment of the invention, theAC/alkali carbonate sorbent material is formulated using activatedcarbons having at least 0.1 cm³/g of mesopore pore volume, preferably upto 2.5 cm³/g or up to cm³/g, or in the range 0.1-3.5 cm³/g, and at least80 m²/g of mesopore pore surface, preferably greater than 100 m²/g,measured using the method described in Example 11.

The particles of the impregnated particulate activated carbon arepreferably essentially spherical, extruded rods, or pellets.

The present invention furthermore relates to a carbon dioxide capturedevice comprising a activated carbon-based material as detailed above,preferably in the form of at least one air permeable containercomprising said impregnated particulate activated carbon in particulateform, most preferably in the form of a multitude of layers of suchcontainers arranged in a stack.

Such a carbon dioxide capture device preferably comprises a housing inwhich the at least one air permeable container containing the particlesof the impregnated particulate activated carbon is located, wherein thehousing has at least one opening for allowing in and/or allowing outatmospheric air for adsorption and closing lids for said at least oneopening to close the housing as well as means for applying a vacuumand/or temperature change for release of the adsorbed carbon dioxide aswell as means for removal of said adsorbed carbon dioxide from thehousing and for collecting and/or further concentrating and/orcondensing the carbon dioxide.

Furthermore the present invention relates to a method for making animpregnated particulate activated carbon material as detailed above,wherein preferably in a first step K₂CO₃ is dissolved in a solvent,preferably water, most preferably deionized water, wherein preferablythe concentration is 1-8 mmol K₂CO₃/ml water, preferably 1.5-4.5 mmolK₂CO₃/ml water, and wherein particulate activated carbon, if need beafter drying and/or purification, is added to the solution, understirring, preferably at a temperature in the range of 5-40° C., mostpreferably at a temperature in the range of 20-30° C., and/or for a timespan in the range of 30 minutes-100 hours, most preferably in the rangeof 6 hours-40 hours, and wherein subsequently at least the solidfraction is isolated and/or dried by (vacuum) evaporation.

The present invention also relates to the use of an impregnatedparticulate activated carbon material as detailed above, preferablyusing a carbon dioxide capture device as detailed above, for capturingcarbon dioxide from atmospheric air, preferably in a cyclic process.

The present invention furthermore relates to a method for capturingcarbon dioxide from atmospheric air using the impregnated particulateactivated carbon as detailed above, preferably a carbon dioxide capturedevice as detailed above, wherein a temperature/vacuum swing cycle, withor without steam injected, is used for adsorption and desorption of thecarbon dioxide.

In such a method preferably at least a part of the desorption of CO₂ isperformed at a pressure in the range of 50-400 mbar_(abs) preferably of100-300 mbar_(abs) and at a temperature in the range of 80-150° C. or atleast a part of the desorption of CO₂ is performed at a pressure in therange of 50-400 mbar_(abs) preferably of 100-300 mbar_(abs) and at atemperature of 35-80° C., preferably of 45-80° C. and another part ofthe desorption of CO₂ is performed at a temperature in the range of80-150° C., preferably of 90-135° C.

Further embodiments of the invention are laid down in the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described in the followingwith reference to the drawings, which are for the purpose ofillustrating the present preferred embodiments of the invention and notfor the purpose of limiting the same. In the drawings,

FIGS. 1(a)-1(d) show N₂ adsorption isotherms for (a) AC mesh 20-40-, (b)AC mesh 4-12, (c) AC extruded, (d) K₂CO₃/AC V (mesh 20-40);

FIG. 2 shows the pore size distributions for AC mesh 20-40 batch 1, ACmesh 4-12, AC extruded;

FIG. 3 shows a BJH plot for AC 1: AC mesh 20-40 batch 1, AC 2: AC mesh20-40 batch 2, AC 3: AC mesh 4-12 and AC 4: AC extruded;

FIG. 4 shows pore size vs. pore volume for AC mesh 20-40 batch 1;

FIG. 5 shows pore size vs. pore volume for AC mesh 20-40 batch 2;

FIG. 6 shows pore size vs. pore volume for AC mesh 4-12;

FIG. 7 shows pore size vs. pore volume for extruded AC;

FIG. 8 shows pore size vs. pore volume for K₂CO₃/AC V;

FIG. 9 shows pore size distributions for AC mesh 20-40 and K₂CO₃/AC mesh20-40;

FIG. 10 shows CO₂ adsorption curves for samples K₂CO₃/AC IV, V, VII,VIII, IX, XI;

FIG. 11 shows CO₂ adsorption curves for K₂CO₃/AC IV after differentdesorption conditions;

FIG. 12 shows CO₂ desorption flow during TVS desorption of 40 g ofK₂CO₃/AC V at 100 mbar and 150° C.

FIG. 13 shows pore size distributions of AC 3:AC mesh 4-12 and AC 7:K₂CO₃/AC mesh 4-12 obtained by mercury porosimetry analysis

FIG. 14 shows pore size distributions of AC2: mesh 20-40 batch 2, AC 3:mesh 4-12 and AC 4: AC extruded obtained with mercury porosimetryanalysis

FIG. 15 shows the correlation between mesopore surface and mesoporevolume of the AC support and the capacity to capture CO₂ of the K₂CO₃/ACsorbent material. AC supports with mesopore surface ≥80 m²/g andmesopore volume ≥0.1 cm³/g are the most apt for formulating the sorbentmaterial

FIG. 16 shows an exemplary adsorption breakthrough curve of thedemonstration plant at an airflow of approximately 1000 m3/h, yieldingan uptake of 1.15 kg of CO₂

FIG. 17 shows an exemplary desorption sorbent and steam temperatures ofthe demonstration plant during desorption by method 1

FIG. 18 shows an exemplary desorption chamber pressure of thedemonstration plant during desorption by method 1

FIG. 19 shows an exemplary concentration and flow of the CO₂-product gasduring desorption on the demonstration plant by method 1

FIG. 20 shows the correlation between the mesopore surface of thepristine activated carbon ingredient and the CO₂ adsorption capacity ofthe sorbent

FIG. 21 shows the cumulative CO₂ adsorption curves for the examinedsorbents

FIG. 22 shows the breakthrough curves for the examined sorbents.

DESCRIPTION OF PREFERRED EMBODIMENTS Example 1. Synthesis of DifferentActivated Carbon/K₂CO₃ Sorbents

Utilized chemicals and materials:

Granular activated carbon, mesh size 20-40 (0.841-0.420 mm) CAS no.:7440-44-0, Sigma Aldrich, DARCO®;

Granular activated carbon mesh size 4-12 (4.76-1.68 mm), CAS no.:7440-44-0, Sigma Aldrich, DARCO®;

Extruded activated carbon 3 mm, CAS no.: 7440-44-0, Cabotcorp, Norit®;

K₂CO₃, CAS no.: 584-08-7, Sigma Aldrich Potassium Cabonate Anhydrous.

Description of the Synthesis of Particulate Activated Carbon/K₂CO₃Sorbents:

K₂CO₃/AC 20-40 Mesh

Method 1: (Batch IV) K₂CO₃ (10.31 g, 0.075 mol) was added to deionizedwater (30 cm³). Granular, mesh 20-40 activated carbon (20.07 g) wasadded to the mixture while stirring. The mixture was left stirring atroom temperature for 24 hours. The mixture was placed on a tray in anoven and heated to 105° C. The material was dried at 105° C. for 3hours.

Method 2: K₂CO₃ (10.0355 g) was added to 60 cm³ of deionized water.Activated carbon 20-40 mesh (20.0577 g) was added to the solution. Themixture was placed in a rotary evaporator and was mixed at roomtemperature for 24 hours. The water was removed at 60° C.

K₂CO₃/AC (Batches V, VII, VIII, IX, XI) were all prepared in the sameway as K₂CO₃/AC IV. Below in Table 1 are the exact amounts of K₂CO₃ andgranular mesh 20-40 activated carbon that were used. K₂CO₃/AC IV, V andVII were produced with activated carbon taken from one supplier batchand K₂CO₃ AC VIII, IX, and XI were taken from a second supplier batch inorder to check reproducibility.

TABLE 1 Mass of K₂CO₃ and AC used in the synthesis of the compositesorbent Batch K₂CO₃ (g) 20-40 mesh activated carbon (g) IV 10.31 20.07 V10.19 20.14 VII 10.13 20.44 VIII 10.12 20.07 IX 10.03 20.09 XI 10.0220.09

K₂CO₃/AC 4-12 mesh: K₂CO₃ (10.05 g, 0.073 mol) was added to deionizedwater (30 cm³). Granular, mesh 4-12 activated carbon (20.01 g) was addedto the mixture while stirring. The mixture was left stirring at roomtemperature for 24 hours. The mixture was placed on a tray in an ovenand heated to 105° C. The material was dried at 105° C. for 3 hours.

K₂CO₃/AC Extruded AC:

Method 1: K₂CO₃ (10.08 g, 0.073 mol) was dissolved in 30 cm³ ofdeionized water. Carbon rods (20.13 g) were placed in a flat beaker andthe aqueous K₂CO₃ solution was pipetted onto it. The activated carbonwas left to soak for 24 hours at room temperature and was then dried inthe oven for two hours at 150° C.

Method 2: K₂CO₃ (10.10 g, 0.073 mol) was dissolved in 30 cm³ ofdeionized water. Activated carbon rods (19.99 g) were added to thesolution. The rods were left to soak for 24 hours at room temperature.The mixture was filtered using gravity filtration and then dried in theoven at 150° C. for two hours. 10 cm³ of the 24 cm³ of filtrate wasdropped onto the dried activated carbon. The mixture was left at roomtemperature for a further 24 hours and was then dried in the oven at150° C. for two hours.

Example 2. Measurement of Tapped Density of Adsorbents

Method for the tapped density measurements:

-   -   1. A graduated cylinder was placed on a PCE (Mettler Toledo)        analytical scale and tarred.    -   2. The graduated cylinder was filled with sorbent particles        using a funnel.    -   3. As the cylinder was filled, it was tapped manually with a        spatula in order to compact the material. The cylinder was        tapped between 10 and 20 times by tapping different parts of the        cylinder, including the base.    -   4. Once the cylinder was filled with 5 cm³ of material, the        weight (g) was recorded.    -   5. The density was calculated by dividing the weight, w by the        volume, V of 5 ml, d=w/V. The error associated with the        measurement is ±0.01 g/ml.

Results for tapped density measurements:

TABLE 2 Tapped densities of AC pure and loaded with K₂CO₃ AC granular20-40 AC granular 20-40 AC granular mesh batch 1/ mesh batch 2/ 4-12mesh/ kg m⁻³ kg m⁻³ kg m⁻³ Pure AC — 403 ± 10 432 ± 10 AC loaded with558 ± 10 592 ± 10 539 ± 10 33 w % K₂CO₃

Example 3. Pore Size, Pore Volume and Specific Surface Area ofAdsorbents

Method for the specific surface area measurements of the sorbents:

Nitrogen adsorption measurements were performed at 77 K on aQuantachrome Autosorb iQ and post-processed using ASiQWin. The mass ofthe sample used was 100 mg, the samples were degassed at 130° C. undervacuum for four hours before measurement.

BET (Brunauer, Emmett and Teller) surface area analysis was done usingthe method described in ISO 9277.

Method for the pore volume and pore size calculations:

The experimental characterization of micro- and macropores is describedin ISO 15901-2 and ISO 15901-3. Micropore and mesopore volume andsurface distributions were calculated using the QSDFT method (quenchedsolid density functional theory) as referred to in Iupac TechnicalReport, Pure Appl. Chem. 2015; 87(9-10): 1051-1069.

The applied calculation tools were:

-   -   “QSDFT—N₂—carbon adsorption branch kernel at 77 K based on a        slit-pore model (pore diameter <2 nm) and cylindrical pore model        (pore diameter >2 nm)” which has an upper calculation limit for        a pore size of 33 nm.    -   “N₂ @ 77K on carbon (slit/cyl./spher. pore)” which contains in        comparison to the kernel above additionally a spherical pore        model for pore sizes above 5 nm and in such allows calculations        up to 50 nm.

The adsorption curves were used instead of the desorption curves due totensile-strength effects.

Results for the specific surface area measurements are given in FIG. 1 .

According to IUPAC definitions micropores are defined as being <2 nm indiameter, mesopores are defined as being 2<x<50 nm in diameter andmacropores are >50 nm. From FIG. 1 (a) it can be seen that AC mesh 20-40has both micro- and mesopores. The knee at low partial pressures showsstrong adsorbate-adsorbent interactions typical of micropores. Thehysteresis indicates the presence of mesopores. K₂CO₃/AC V in FIG. 1 (d)has the same adsorption isotherm with a lower volume, which indicatesthat both micro- and mesopores were filled with K₂CO₃. The AC mesh 4-12also has a similar isotherm showing that this material too has micro-and mesopores. From FIG. 1 (c) it can be seen that the extruded AC haspredominantly micropores and few mesopores with small size featuring atypical type 1 adsorption isotherm. From the pore size distributionanalysis below (see FIG. 2 ) one can see that the extruded AC haspredominantly pores in the size range of <2 nm or <5 nm of porediameter.

The specific surface areas are summarized in Table 3 below:

TABLE 3 BET surface areas of pristine AC supports and K₂CO₃/AC V AC20-40 AC 20-40 AC 4-12 mesh batch 1/ mesh batch 2/ mesh AC extruded/ m²g⁻¹ m² g⁻¹ m² g⁻¹ m² g⁻¹ Pure AC 585 650 448 757 AC loaded with 304 — —— 33 w % K₂CO₃

The AC extruded has the highest surface area as this material haspredominantly micropores and small mesopores below 5 nm or below 2 nm.AC mesh 4-12 has the lowest surface area indicating that it presumablyhas the largest pore sizes. As shown below the extruded AC has thelowest CO₂ capacity for CO₂ capture from air and AC mesh 4-12 has thehighest, so that the AC support with the lowest specific surface areaperforms best. This is contrary to prior art, where it is largelyclaimed that the specific surface area of the support needs to bemaximized, but no indications with respect to pore size are given.

The surface area for the two AC mesh 20-40 supplier batches differ by10%. The fact that the surface area in batch 1 is lower than that ofbatch 2 could indicate that it has larger pore sizes, e.g. more porevolume in size above 30 nm.

The results for the pore volume and pore size calculations using the twodifferent kernels are shown in Table 4a and Table 4b below:

TABLE 4a Surface area and pore volumes for micropores and mesopores asdetermined by QSDFT calculations with kernel “QSDFT - N2 - carbonadsorption branch kernel at 77K based on a slit-pore model (porediameter < 2 nm) and cylindrical pore model (pore diameter > 2 nm)”Micropores Mesopores Micropore Micropore Mesopore Mesopore surface area/volume/ surface area/ volume/ Sample ID m² g⁻¹ cm³ g⁻¹ m² g⁻¹ cm³ g⁻¹ ACmesh 375 0.144 210 0.408 20-40 batch 1 AC mesh 402 0.163 248 0.498 20-40batch 2 AC mesh 4-12 315 0.130 133 0.274 AC extruded 660 0.270 97 0.076K₂CO₃/AC V 188 0.073 116 0.258

TABLE 4b Surface area and pore volumes for pore sizes as determined byQSDFT calculations with kernel “N2 @ 77K on carbon (slit/cyl./spher.pore)” Micropores Mesopores Micropore Micropore Mesopore Mesoporesurface area/ volume/ surface area/ volume/ Sample ID m² g⁻¹ cm³ g⁻¹ m²g⁻¹ cm³ g⁻¹ AC mesh 381 0.146 191 0.383 20-40 batch 1 AC mesh 401 0.163215 0.471 20-40 batch 2 AC mesh 4-12 304 0.121 126 0.261 AC extruded 6610.271 59 0.073 K₂CO₃/AC V 188 0.073 299 0.243

Analysis of Table 4a and Table 4b shows that both kernels yieldedcomparable results which is why in the following analysis is limited toresults obtained with kernel “QSDFT—N₂—carbon adsorption branch kernelat 77 K based on a slit-pore model (pore diameter <2 nm) and cylindricalpore model (pore diameter >2 nm)” (except for BJH calculation below).

By comparing the pore volume of pristine AC mesh 20-40 batch 1 andK₂CO₃/AC V it can be seen that roughly 50% of the micropore volume wasfilled with K₂CO₃ as well as around 35% of the mesopore volume, hence,both pore regimes contribute to K₂CO₃ modification, as further explainedbelow. Due to the much higher specific surface area of the microporevolume (375 m²/g) than the mesopore volume (210 m²/g) in AC mesh 20-40batch 1 it can be assumed that the K₂CO₃ filling is distributed on abigger surface in the micropores and in such offering higher masstransfer area compared to the mesopore volume. However, as describedbelow especially the pore volume available at pore size above 5 nm,hence, medium to large size mesopores as well as small macropores,contribute to a favourable K₂CO₃ modified sorbent for CO₂ capture fromair. This finding has not been described in prior art, rather theopposite.

As described further below K₂CO₃/AC V (AC mesh 40-20 batch 1) andK₂CO₃/AC 4-12 mesh feature high CO₂ capacities where K₂CO₃/AC extrudedshows very little capacity for CO₂ capture from air.

From the adsorption isotherms, specific surface area, pore volume andpore size distributions data (see FIG. 2 ) it can be seen that ACextruded is predominantly microporous and has few pores in the smallrange below 2 nm or below 5 nm. This support did not work for CO₂capture from air and so it can be concluded that pore sizes >2 nm or >5nm are needed to provide feasible supports for CO₂ capture from air.

In addition to the BET and DFT calculations BJH (Barrett-Joyner-Halenda)calculations were made. The comparisons for AC mesh 20-40 batch 1, ACmesh 20-40 batch 2, AC mesh 4-12 and AC extruded are shown in FIG. 3 .It can be seen that AC mesh 20-40 batch 2 has the highest pore volumefollowed by AC mesh 20-40 batch 1 and then AC mesh 4-12. There seems tobe a peak at approximately 35 nm for the materials AC mesh 20-40 batch 1and mesh 20-40 batch 2. AC mesh 4-12 has a peak of pore volume around 45nm. FIG. 3 seems to indicate that the best performing materials do havemesopores.

FIGS. 4-8 show the detailed measurement results of the cumulative porevolume versus the pore size (given as half pore width), and FIG. 9 givesthe pore size distribution for the pristine AC mesh 20-40 batch 1 andthe K₂CO₃/AC V.

From FIG. 9 it can be seen that the pores in the range 5<x<30 nm are allfilled approximately equally with K₂CO₃. There isn't a specific rangethat seems to take up more K₂CO₃.

Example 4. CO₂ Adsorption/Desorption Capacities of Adsorbents

The method used to determine the CO₂ adsorption/desorption capacity wasas follows:

-   -   1. The as synthesized material was weighed. It was placed on a        tray and heated to 150° C. in a Binder natural convection oven.    -   2. The material was desorbed for 2 hours once the oven reached        150° C.    -   3. After two hours the oven was cooled and once a temperature of        80° C. was reached the material was removed and weighed.    -   4. 6 g of desorbed sample was filled into a cylinder with an        inner diameter of 40 mm and a height of 40 mm and placed into a        CO₂ adsorption/desorption device, where it was exposed to a flow        of 2.0 NL/min of air at 30° C. containing 450 ppmv CO₂, having a        relative humidity of 60% corresponding to a temperature of        30° C. for a duration of 1000 min. The amount of CO₂ adsorbed on        the adsorbent was determined by integration of the signal of an        infrared sensor measuring the CO₂ content of the air stream        leaving the cylinder. After CO₂ adsorption the adsorbent was        weighed again.

The results of the CO₂ adsorption/desorption measurements are summarizedin the FIGS. 10 and 11 and Table 5.

TABLE 5 CO₂ capacities after 1000 min. and 180 min. for differentbatches of K₂CO₃/AC CO₂ capacity after CO₂ capacity after 1000 min.adsorption/ 180 min. adsorption/ mmol g⁻¹ mmol g⁻¹ AC mesh 20-40 batch 1IV 1.350 0.641 V 1.528 0.760 VII 1.330 0.673 AC mesh 20-40 batch 2 VIII1.215 0.676 IX 1.382 0.739 XI 1.328 0.586 Average 1.4 ± 0.1 0.68 ± 0.06

The average CO₂ capacity at 30° C., 60% relative humidity (at 30° C.)and 450 ppmv CO₂ concentration after 1000 min. adsorption measuredacross six samples produced from two activated carbon mesh 20-40 batcheswas found to be 1.4±0.1 mmol/g. The average capacity after 180 min.adsorption was found to be 0.68±0.06 mmol/g.

In order to identify the threshold desorption temperature the materialwas desorbed at different temperatures. Once the set temperature wasreached, the material was left desorbing for two hours. After two hoursthe oven was left to cool until it reached 80° C. The material was thenremoved and tested with above described CO₂ adsorption protocol for CO₂adsorption capacity.

The threshold temperature is concluded to be 115° C. because afterdesorbing at this temperature the total CO₂ capacity as described aboveis reached. When the material is desorbed at 110° C., the adsorptioncapacity was 1.02 mmol g⁻¹, indicating it was not fully desorbed.

The CO₂ capacity at the CO₂ adsorption conditions described above forthe K₂CO₃ impregnated on 4-12 mesh activated carbon was found to be1.687 mmol/g.

The CO₂ capacity at the CO₂ adsorption conditions described above forthe K₂CO₃ impregnated extruded carbon was found to be 0.100 and 0.219mmol/g for the two syntheses, hence, it is too little to qualify asfeasible adsorbent for CO₂ capture from air.

Example 5. Temperature-Vacuum-Swing Desorption of Adsorbents

For commercial application of the adsorbents described herein desorptionmethods are required which produce a concentrated stream of CO₂ duringdesorption. Temperature-vacuum-swing (TVS) desorption is a desorptiontechnique used mainly for amine-modified adsorbents for theirregeneration. In this example we tested whether TVS desorption is alsofeasible for desorption of K₂CO₃ modified activated carbons.

The method for the TVS desorption was as follows:

-   -   1. The adsorbent was desorbed in the oven at 150° C. and 40        grams were weighed out for the experiment.    -   2. The 40 g of dry mass was filled into a rectangular CO₂        adsorption/desorption chamber having inner dimensions of 62        mm×62 mm×72 mm.    -   3. For TVS desorption the chamber containing the adsorbent was        heated by an external source to 150° C. and the pressure was        reduced to 100 mbar with a vacuum pump for a duration of 300        min.    -   4. After the desorption, the chamber is cooled to 30° C. and        once this temperature was reached CO₂ adsorption was performed        with air at a temperature of 30° C., a flow rate of 15.0 NL/min,        a CO₂ concentration of 450 ppmv, a relative humidity of 60% at        30° C. for a duration of 1000 min.    -   5. The adsorption and desorption were repeated for 5 consecutive        cycles.

FIG. 12 shows the CO₂ desorption flow during TVS desorption of 40 g ofK₂CO₃/AC V at 100 mbar and 150° C. Integration of the signal of the flowmeasurement device yielded a CO₂ desorption capacity of 1.3 mmol CO₂/gand subsequent CO₂ adsorption yielded 1.2 mmol CO₂/g, consequentlyconfirming that the adsorbent can be regenerated with TVS desorption.

Example 6. Mercury Porosimetry Measurements

Mercury porosimetry measurements were performed to analyze the poresizes and pore volumes not accessible through N₂ adsorption measurements(see Example 3). In order to perform mercury porosimetry measurementsthe ISO 15901-1 measurement standard was followed and the followingparameters were used:

-   -   Mercury surface tension: 0.48 N/m    -   Mercury contact angle: 150°    -   Test method: PASCAL (Pressurized by Automatic Speed-up and        Continuous Adjustment Logic)    -   Max. pressure: 400 MPa    -   Increase speed: 6-19 MPa/min    -   Preparation: Degassing for 30 min. (also ensured <0.03 kPa        reached)

The results of Hg porosimetry analysis are summarized the followingtable:

TABLE 6 Results obtained with Hg porosimetry analysis Total pore volume/Pore volume Sample ID cm³ g⁻¹ 50-1′000 nm/cm³ g⁻¹ AC mesh 20-40 batch 20.65 0.23 AC mesh 4-12 0.62 0.25 AC extruded 0.29 0.03 K₂CO₃/AC mesh20-40 0.38 0.06 batch 2 K₂CO₃/AC mesh 4-12/ 0.29 0.08 K₂CO₃ K₂CO₃/ACextruded/ 0.24 0.03 K₂CO₃

FIG. 13 shows the pore size distributions for AC mesh 4-12 and K₂CO₃/ACmesh 4-12. It can be seen that the pore volume over the completemeasurement range 3-120'000 nm is reduced for the material modified withK₂CO₃, indicating that all pores sizes contribute to K₂CO₃ modification.It can be further seen that the pore volume reduction after K₂CO₃impregnation in the pore size range of 20-1'000 nm is the largest,indicating that this is the most feasible pore size range for activatedcarbon adsorbents modified with K₂CO₃ used for CO₂ capture from air.

FIG. 14 summarizes the pore size distributions for AC mesh 20-40, ACmesh 4-12 and AC extruded. It can be seen that AC mesh 20-40 and AC mesh4-12 have very different pore size distributions than AC extruded. ACextruded contains mostly micropores and macropores in the size rangeabove 1000 nm. Such pore sizes are not favourable for activated carbonsmodified with K₂CO₃ for CO₂ capture from air. In turn pore sizes in therange of 5-1'000 nm, as present in AC mesh 20-40 and AC mesh 4-12 aresuitable for activated carbons modified with K₂CO₃ to be used for CO₂capture from air.

Example 7: Synthesis of Activated Carbon/Na₂CO₃/K₂CO₃ Sorbents (See AlsoMore Detailed Sorbent Production Further Below)

Utilized chemicals and materials:

Extruded activated carbon, AC 12 (Table 10).

K₂CO₃, CAS no.: 584-08-7, Sigma Aldrich Potassium Cabonate Anhydrous

Na₂CO₃, Cas no: 497-19-8, Sigma Aldrich, Sodium Carbonate Anhydrous

Description of the Synthesis of Extruded Activated Carbon Impregnatedwith Na₂CO₃/K₂CO₃

K₂CO₃ and Na₂CO₃ (total 20 g) were dissolved in 120 ml of water, and 40g of activated carbon was added to the solution and soaked overnight,after which the water was removed at 100 mbar and 60° C. The resultingsamples were dried in oven at 150° C. for 2 hours. The synthesis wasrepeated by using different relative weight proportions of the sodiumand potassium salts. The materials thus prepared contained 10, 30 and50% wt. of the sodium salt with respect to the potassium salt.

Example 8: Temperature Swing Adsorption Desorption Cycles with Air PurgeUsing AC/Na₂CO₃/K₂CO₃ Sorbents

The samples prepared in example 7 were tested using five cycles ofadsorption-desorption as described in the following: 6 g of eachmaterial where placed in a steel vessel delimited by a steel net throughwhich a controlled air flow of 2 l/min was passed. The air flowedthrough the samples had a controlled CO₂ concentration of 450 ppmv. Theoutgoing air flow was controlled for the CO₂ concentration using a CO₂infrared sensor. During desorption, the desorption bed temperature wasset to 94° C., and a constant air flow of 2 l/min was applied.Adsorption and desorption times were 300 minutes and 120 minutesrespectively. Surprisingly cyclic adsorption capacities increased up tofour times by using a mixture of Na and K carbonates, with respect tothe adsorption capacities of the pure potassium carbonate salt, as shownin table

TABLE 7 Cyclic air purge adsorption capacities using mixed Na and Kcarbonates Mixture ratio Cyclic air purge adsorption capacity [% wt.Na₂CO₃:K₂CO₃] [mmol/g]  0:100 0.12 10:90 0.31 30:70 0.39 50:50 0.49

Example 9: Temperature-Vacuum-Swing Adsorption Desorption Cycle withAC/Na₂CO₃/K₂CO₃ Sorbents

The TVS experiment of example 5 was repeated, with the followingmodifications: 120° C. of jacket temperature during the desorptionphase; adsorption and desorption times of 300 minutes; material asprepared in Example 7; four consecutives adsorption and desorptioncycles were applied. The results in table 8 show an increase of 50% ofcyclic capacity of CO₂ for the material prepared with the combination30-70 w % Na₂CO₃—K₂CO₃ as compared to the same material but onlyimpregnated with K₂CO₃ in the same total weight proportion.

TABLE 8 cyclic TVS adsorption capacities using mixed Na and K carbonatesMixture ratio Cyclic TVS adsorption capacity [% wt. Na₂CO₃:K₂CO₃][mmol/g]  0:100 0.40 10:90 0.49 30:70 0.60 50:50 0.42

Example 10: Use of Na/K Carbonate Mixtures to Decrease the RegenerationTemperature of a Sorbent System

The TVS experiment of example 9 was repeated with the modificationsdescribed in the following. The material described in example 7 wasimpregnated with K₂CO₃ and used in a TVS experiment applying adesorption (jacket) temperature of 140° C.; another experiment wascarried out using the material described in example 7 impregnated with amixture of 30:70% wt. of Na/K carbonates, and using a desorption(jacket) temperature of 120° C. The results shown in table 9 show thatthe material impregnated with a mixture of Na and K carbonates afford a20% increase in cyclic capacity at 20° C. lower (jacket) temperaturewith respect to the same material impregnated with the pure K₂CO₃.

TABLE 9 cyclic TVS adsorption capacities using pure and mixed Na and Kcarbonates at two different desorption temperatures Cyclic TVSadsorption Cyclic TVS adsorption capacity [mmol/g] at capacity [mmol/g]at Mixture ratio 140° C. desorption 120° C. desorption [% wt.Na₂CO₃:K₂CO₃] temperature temperature  0:100 0.50 0.40 30:70 0.60 0.60

Example 11: Correlation Between Mesopore Volume and Mesopore Surface ofthe AC and the CO₂ Capture Capacity of the K₂CO₃/AC Sorbent

K₂CO₃/AC sorbents were prepared according to method described in Example1, method 2, using as ingredients the activated carbons AC 1 to AC 18listed in table 10. The pores size distribution and the mesopore volumeand mesopore surface of the ACs were measured by nitrogen physisorptionaccording to the method described in Example 3 and with the followingspecificities: prior to the analysis, the samples were outgassed at 150°C. for 12 hours under vacuum. For the calculation of the micro- andmesopore surface area contributions, the t-plot method was appliedaccording to ISO 15901-3. The adsorption branch of the isotherm was usedfor the calculation. The CO₂ capture capacities of the resultingK₂CO₃/AC sorbents were measured according to the method alreadydescribed in Example 4. The CO₂ capture capacities of the sorbents wereplotted against the mesopore volume and the mesopore surface, revealingthat the sorbents that were prepared using AC that had a mesopore volume≥0.1 cm³/g, and/or a mesopore surface ≥80 m²/g showed superiorcapacities of CO₂ capture, measurable as ≥0.8 mmol/g within theexperimental setup already described, while the sorbent materials thatwere prepared using ACs that had a mesopore volume ≤0.1 cm³/g, and/or amesopore surface ≤80 m²/g showed less pronounced capacities of CO₂capture, measurable as ≤0.8 mmol/g, and more often ≤0.4 mmol/g withinthe experimental setup already described. The plot of reference is shownin FIG. 15 . The optimal loading of the sorbent was not optimized,nonetheless a clear trend can be read in the plotted values.

TABLE 10 Properties of the activated carbons used for example 11 Corre-V_(pore) ≥ S ≥ 2- sponding CO₂ V_(pore) < 2-50 S < 2 50 capacity ofV_(pore) 2 nm nm nm nm S_(BET) Particle size modified AC No. [cm³/g][cm³/g] [cm³/g] [m²/g] [m²/g] [m²/g] and type [mmol/g] AC 1 0.38 0.340.04 878 33 911 3 mm, 0.15 extruded AC 2 0.358 0.323 0.035 798 13 811 3mm, 0.47 extruded AC 3 0.683 0.578 0.105 1256 53 1309 4 mm, 0.48extruded AC 4 0.425 0.383 0.042 973 39 1012 3 mm, 0.39 extruded AC 50.388 0.339 0.049 798 31 829 3 mm, 0.38 extruded AC 6 0.355 0.31 0.045704 26 730 4 mm, pellet 0.09 AC 7 0.413 0.371 0.042 909 22 931 3 mm,0.14 extruded AC 8 0.673 0.538 0.135 1128 107 1235 4 mm, 0.9 extruded AC9 0.312 0.083 0.229 172 100 272 2 mm, 0.86 pellets AC 0.526 0.114 0.412272 185 457 4-12 mesh, 1.4 10 granular AC 0.944 0.155 0.789 367 390 75720-40 mesh, 1.9 11 granular AC 0.712 0.588 0.124 1425 98 1523 0.8 mm,1.1 12 extruded AC 0.788 0.639 0.149 1531 134 1665 0.8 mm, 1.46 13extruded AC 0.629 0.406 0.223 874 129 1003 8-30 US 1.49 14 mesh,granular (0.6-2.36 mm) AC 0.604 0.503 0.101 1245 79 1324 0.5-0.8 mm, 1.415 spherical AC 0.445 0.289 0.156 723 119 842 8-30 US 1.57 16 mesh,granular (0.6-2.36 mm) AC 0.606 0.292 0.314 711 207 918 8-30 US 1.53 17mesh, granular (0.6-2.36 mm) AC 0.496 0.32 0.176 770 106 876 12-40 US1.95 18 mesh, granular (0.425-1.7 mm)

Example 12. Sorbent and its Production

FIG. 20 shows the correlation between the mesopore surface of thepristine activated carbon ingredient and the CO₂ adsorption capacity ofthe sorbent.

We hereby show that for activated carbon supports impregnated with K₂CO₃the CO₂ adsorption capacities from ambient air increases with increasingmesopore surface. In particular activated carbons having mesoporesurfaces above 80 m2/g are especially apt as formulation ingredient. Therange of 80-600 m2/g, and most preferably 80-400 m2 g, is optimum forthe mesopore surface of the activated carbon ingredient used in theformulation of the sorbent as described further below.

To obtain a good sorbent it is important that the K₂CO₃-solution entersthe pores of the support and infiltrates the large internal surfacearea. Capillary forces can be assumed to be a major driving force forthis process. However, before the infiltration the pores are usuallyfilled with air that needs to be replaced and that takes time to diffuseout of the porous support. Usually, rather long soaking times >12 h areused for the impregnation of activated carbon with K₂CO₃. Theinfiltration can be accelerated by the application of vacuum to removethe entrapped air followed by a return to atmospheric pressure to pushthe solution into the pores.

The application of a vacuum of 100 mbar for 2 times 5 min followed by areturn to ambient pressure at the beginning of a 3 h impregnation periodallowed improving the CO₂-adsorption capacity of the sorbent by a factorof 2.4 for a 180-min adsorption experiment. To be specific the capacityraised from 0.37 mmol/g to 0.88 mmol/g. The performance of the sorbentproduced within 3 h with initial vacuum is similar to a sorbent producedby impregnating 18 h without initial vacuum. The latter one showed anadsorption capacity of 0.84 mmol/g. The differences between the two canbe regarded to be within the experimental error.

FIG. 21 shows the cumulative CO₂ adsorption curves for the examinedsorbents

The same trend can be observed in the break through curves shown in FIG.22 . The two curves of the sorbent prepared in 3 h with initial vacuumand the one of the sorbent prepared in 18 h without initial vacuum arealmost identical and did not reach breakthrough within 180 minadsorption time. In contrast, the sorbent prepared in 3 h withoutinitial vacuum showed a much steeper breakthrough curve reaching 90% ofthe inlet CO₂ concentration already in less than 2 h.

FIG. 22 shows the breakthrough curves for the examined sorbents

From the data shown it can be concluded that applying vacuum for a shortperiod of time followed by returning to ambient pressure at thebeginning of the impregnation of a porous support material with asolution of an active phase is effective in drastically reducing thenecessary impregnation time. For a sorbent based on activated carbon andK₂CO₃ we could realize a reduction of the soaking time by a factor ofsix. This is highly improving the state-of-the art preparation of suchmaterials, reducing drastically the production time and costs andsimultaneously increasing the production capacity of a given facility.

Sorbent Preparation:

To prepare 30 g dry sorbent 10 g K₂CO₃ (Sigma Aldrich, analytical grade)were dissolved in a 11 pear flask in 60 ml de-ionized water by mildlyshaking it. To the clear solution 20 g activated carbon (Sigma Aldrich,DARCO 20-40 mesh) were added. The flask was mounted on a rotaryevaporator (Heidolph) and rotated at approx. 30 rpm for 3 or 18 h asrequired. For the sample with initial vacuum the apparatus was evacuatedto approx. 100 mbar followed by venting to ambient pressure with air.This step was repeated once. The soaking was done in any case at ambientpressure. After the required impregnation time, the flask was lowered ina water bath at 60° C. and vacuum was applied at approx. 100 mbar toevaporate the water. After 30-45 min the dried sorbent was collectedfrom the flask and stored in a plastic bottle until further use.

Sorbent Characterization:

Prior to the measurement of the adsorption characteristics the sorbentwas spread on a tray and placed in an oven (Binder) at 150° C. for atleast 2 h to desorb any pre-adsorbed CO2 or H2O.

Immediately afterwards 6 g of the sorbent (assumed solid content: 100%)were weighed into the measurement cell and transferred into the reactorof the RC testing unit. The reactor was sealed immediately to avoid CO₂or H₂O take-up from air and the adsorption measurement was started usingthe following parameters:

Adsorption flow: 2 Nl/min; Adsorption temperature: 30° C.; CO₂concentration adsorption gas: 450 ppm; Relative Humidity: 60% rH (30°C.).

The invention claimed is:
 1. A method for making a particulate activatedcarbon material for capturing CO₂ from air, wherein the particulateactivated carbon is impregnated with two different alkali carbonatesalts: K₂CO₃ and Na₂CO₃, wherein the alkali carbonate salt with thesmallest weight proportion is present in an amount of at least 5 weight% with respect to the total of an impregnating mixture of the two alkalicarbonate salts, wherein said two alkali carbonate salts are dissolvedin a solvent, wherein pristine particulate activated carbon having aspecific surface area, determined using nitrogen adsorption methods asdescribed in ISO 15901-2 and ISO 15901-3 and using a t-plot method, ofat least 80 m²/g in the pore size range of more than 2 nm to at most 50nm, is added to form a suspension, and wherein subsequently at least asolid fraction is isolated, dried by evaporation, or both, to obtain theimpregnated particulate activated carbon.
 2. The method according toclaim 1, wherein the solvent is water.
 3. The method according to claim1, wherein the pristine particulate activated carbon has a pore volumeof at least 0.1 cm3/g in the pore size range more than 2 nm to at most50 nm, or wherein the pristine particulate activated carbon has aspecific surface area of 80-600 m2/g, in the pore size range of morethan 2 nm to at most 50 nm, in each case determined using nitrogenadsorption methods according to ISO 15901-2 and ISO 15901-3 andaccording to the t-plot method.
 4. The method according to claim 1,wherein the suspension is subjected to at least one time period withreduced pressure, and wherein before isolation of the impregnatedparticulate activated carbon the suspension is returned to ambientpressure for a time period of at least 60 seconds.
 5. A particulateactivated carbon material for capturing CO₂ from air, wherein theparticulate activated carbon is impregnated with two different alkalicarbonate salts: K₂CO₃ and Na₂CO₃, wherein the alkali carbonate saltwith the smallest weight proportion is present in an amount of at least5 weight % with respect to a total of an impregnating mixture of thealkali carbonate salts, wherein at least one of the following conditionsapplies: the impregnated particulate activated carbon has, determinedusing nitrogen adsorption methods as described in ISO 15901-2 and ISO15901-3 and according to the QSDFT calculation scheme, a) a pore volumeof at least 0.10 cm³/g for pore sizes of at least 5 nm and a pore volumeof at most 0.30 cm³/g for pore sizes of less than 5 nm; b) a pore volumeof at least 0.04 cm³/g for pore sizes of at least 2 nm and a pore volumeof at most 0.35 cm³/g for pore sizes of less than 2 nm; c) a poresurface of at least 20 m2/g for pore sizes above 2 nm or in the range of2-50 nm.
 6. The material according to claim 5, wherein the alkalicarbonate salt impregnated particulate activated carbon has, a porevolume of at least 0.1 cm³/g for pore sizes above 5 nm or in the rangeof 5-50 nm, determined using nitrogen absorption methods as described inISO 15901-2 and ISO 15901-3 according to the QSDFT calculation scheme,or a pore volume of at least 0.05 cm³/g for pore sizes in the range of50-1,000 nm, as determined using mercury porosimetry analysis asdescribed in ISO 15901-1, or a pore volume of at most 0.25 cm³/g or atmost 0.2 cm³/g or in the range of 0.05-0.2 cm³/g or 0.05-0.15 cm³/g, forpore sizes of less than 5 nm, determined using nitrogen absorptionmethods as described in ISO 15901-2 and ISO 15901-3 according to theQSDFT calculation scheme, or wherein the alkali carbonate saltimpregnated particulate activated carbon has, determined using nitrogenadsorption methods as described in ISO 15901-2 and ISO 15901-3 andaccording to the QSDFT calculation scheme, a pore surface of at least 20m²/g for pore sizes above 5 nm or in the range of 5-50 nm, or a poresurface of at most 500 m²/g or in the range of 150-500 or 100-400 m²/gfor pore sizes of less than 5 nm.
 7. The material according to claim 5,wherein the impregnated particulate activated carbon has a BET surfacearea according to ISO 9277 in the range of 100-800 m²/g, or of less than500 m²/g or wherein the impregnated particulate activated carbon has atapped density in the range of 300-800 kg/m³, or wherein the impregnatedparticulate activated carbon has a particle size in the range of 0.1-8mm, or in the range of mesh (ASTM) 3-140.
 8. The material according toclaim 1, wherein the impregnated particulate activated carbon containsat least 10% by weight of alkali carbonate salt.
 9. The materialaccording to claim 5, wherein the alkali carbonate salt impregnatedparticulate activated carbon has an average carbon dioxide capacity, at30° C., 60% relative humidity and 450 ppmv carbon dioxide concentrationafter 1000 minutes adsorption in the range of 0.5-5 mmol/g, or whereinthe particles of the alkali carbonate salt impregnated particulateactivated carbon are essentially spherical, extruded rods, pellets. 10.A carbon dioxide capture device comprising a material according to claim5.
 11. The carbon dioxide capture device according to claim 10, whereinit comprises a housing in which the at least one air permeable containercontaining the particles of the alkali carbonate salt impregnatedparticulate activated carbon is located, wherein the housing has atleast one opening for allowing in and/or allowing out atmospheric airfor adsorption and closing lids for said at least one opening to closethe housing as well as means for applying a vacuum and/or temperaturechange for release of the adsorbed carbon dioxide as well as means forremoval of said adsorbed carbon dioxide from the housing and forcollecting and/or further concentrating and/or condensing the carbondioxide.
 12. A method of using a material according to claim 5 forcapturing carbon dioxide from atmospheric air.
 13. A method forcapturing carbon dioxide from atmospheric air using the materialaccording to claim 5, wherein a temperature swing cycle or atemperature/vacuum swing cycle, with or without steam injected, is usedfor adsorption and desorption of the carbon dioxide.
 14. The methodaccording to claim 13, where at least a part of the desorption of CO₂ isperformed at a pressure in the range of 50-400 mbar_(abs) and at atemperature in the range of 80-150° C. or where at least a part of thedesorption of CO₂ is performed at a pressure in the range of 50-400mbar_(abs) and at a temperature of 35-80° C. and another part of thedesorption of CO₂ is performed at a temperature in the range of 80-150°C.
 15. The method according to claim 1 wherein said pristine particulateactivated carbon is at least one of dried and purified before use. 16.The method according to claim 1, wherein the solvent is deionized water,and wherein the concentration of the alkali carbonate salt is 1-8 mmol(total) alkali carbonate salt per ml water, or 1.5-4.5 mmol/m1 water, orwherein as pristine particulate activated carbon a pristine,non-oxidized, particulate activated carbon is added to the solutionunder stirring, at a temperature in the range of 5-40° C., or at atemperature in the range of 20-30° C., or for a time span in the rangeof 30 minutes-100 hours, or in the range of 6 hours-40 hours, andwherein subsequently at least the solid fraction is isolated and/ordried by evaporation, including vacuum evaporation.
 17. The methodaccording to claim 1, wherein the pristine particulate activated carbonhas a pore volume of at least 0.1 cm3/g and at most 2.5 cm3/g in thepore size range more than 2 nm to at most 50 nm, or wherein the pristineparticulate activated carbon has a specific surface area of 80-400 m2/gin the pore size range of more than 2 nm to at most 50 nm, in each casedetermined using nitrogen adsorption methods according to ISO 15901-2and ISO 15901-3 and according to the t-plot method.
 18. The methodaccording to claim 1, wherein the suspension is subjected to at leastone time period with a vacuum of at most 300 mbar, or at most 200 mbar,or in the range of 10-150 mbar, wherein that reduced pressure timeperiod is at least 60 seconds, or at least 2 minutes, or 3-20 or 5-10minutes, and wherein before isolation of the impregnated particulateactivated carbon the suspension is returned to ambient pressure for atime period of at least 60 seconds, wherein at least two such cyclesincluding a time period of reduced pressure of at least 60 seconds, orof at least 2 min, are used and a following time period of at least 60seconds, or of at least 2 min, of ambient pressure is carried out, andwherein the total impregnation period before isolation can be in therange of 2-5 hours, or in the range of 2.5-3.5 hours.
 19. Theparticulate activated carbon material according to claim 5, wherein saidmaterial is prepared using pristine particulate activated carbon, havinga specific surface area, determined using nitrogen adsorption methods asdescribed in ISO 15901-2 and ISO 15901-3 and using a t-plot method, ofat least 80 m²/g in the pore size range of more than 2 nm to at most 50nm, added to a solution of K₂CO₃ and Na₂CO₃.
 20. The particulateactivated carbon material according to claim 5, wherein the impregnatedparticulate activated carbon has, determined using nitrogen adsorptionmethods as described in ISO 15901-2 and ISO 15901-3 and according to theQSDFT calculation scheme, a pore surface in the range of 40-250 or45-200 m2/g for pore sizes above 2 nm or in the range of 2-50 nm. 21.The particulate activated carbon material according to claim 5, whereinthe alkali carbonate salt impregnated particulate activated carbon has:a pore volume in the range of 0.1-2.2 or 0.2-1.5 cm³/g for pore sizesabove 5 nm or in the range of 5-50 nm, determined using nitrogenabsorption methods as described in ISO 15901-2 and ISO 15901-3 accordingto the QSDFT calculation scheme, or a pore volume in the range of0.05-0.2 cm³/g or 0.05-0.15 cm³/g, for pore sizes of less than 5 nm,determined using nitrogen absorption methods as described in ISO 15901-2and ISO 15901-3 according to the QSDFT calculation scheme, or whereinthe alkali carbonate salt impregnated particulate activated carbon has,determined using nitrogen adsorption methods as described in ISO 15901-2and ISO 15901-3 and according to the QSDFT calculation scheme, a poresurface in the range of 20-500 or 50-400 m²/g for pore sizes above 5 nmor in the range of 5-50 nm, or a pore surface in the range of 150-500 or100-400 m²/g for pore sizes of less than 5 nm.
 22. The particulateactivated carbon material according to claim 5, wherein the impregnatedparticulate activated carbon has a tapped density in the range of400-600 kg/m³, or wherein the impregnated particulate activated carbonhas a particle size in the range of 0.5-1.5 mm, or in the range of mesh(ASTM) 4-50.
 23. The particulate activated carbon material according toclaim 5, wherein an impregnating mixture of said two alkali carbonatesalts is consisting of K₂CO₃ as well as Na₂CO₃, in a weight ratio ofK₂CO₃ to Na₂CO₃ in the range of 95:5-5:95, or in the range of90:10-10:90, or in the range of 40:60-95:5.
 24. The particulateactivated carbon material according to claim 1, wherein the impregnatedparticulate activated carbon contains at least 20% by weight or at least30% by weight, or in the range of 25-45% by weight of K₂CO₃.
 25. Theparticulate activated carbon material according to claim 5, wherein thealkali carbonate salt impregnated particulate activated carbon has anaverage carbon dioxide capacity, at 30° C., 60% relative humidity and450 ppmv carbon dioxide concentration after 1000 minutes adsorption inthe range of 1-2.5 mmol/g or after 180 minutes adsorption in the rangeof 0.5-2 mmol/g or in the range of 0.6-1.5 mmol/g or wherein theparticles of the alkali carbonate salt impregnated particulate activatedcarbon are essentially spherical, extruded rods, or pellets.
 26. Acarbon dioxide capture device comprising a material according to claim5, in the form of at least one air permeable container comprising saidmaterial in particulate form, including in the form of a multitude oflayers of such containers arranged in a stack.
 27. The method accordingto claim 12 comprising using a carbon dioxide capture device forcapturing carbon dioxide from atmospheric air, in a cyclic process, saiddevice having said particulate activated carbon material.
 28. A methodfor capturing carbon dioxide from atmospheric air using a carbon dioxidecapture device having the material according to claim 5, wherein atemperature swing cycle or a temperature/vacuum swing cycle, with orwithout steam injected, is used for adsorption and desorption of thecarbon dioxide.
 29. The method according to claim 13, where at least apart of the desorption of CO₂ is performed at a pressure in the range of100-300 mbar_(abs) and at a temperature in the range of 80-150° C., orwhere at least a part of the desorption of CO₂ is performed at apressure in the range of 100-300 mbar_(abs) and at a temperature of45-80° C. and another part of the desorption of CO₂ is performed at atemperature in the range of 90-135° C.